A printing apparatus is employed widely as the information output means of printers, copiers and facsimile machines. A thermal printer is one example of such a printing apparatus. Another example is an ink-jet printing apparatus that prints characters and images by discharging ink on a printing medium such as printing paper.
An ink-jet printing apparatus prints by discharging ink while a printhead that discharges the ink is made to move relative to the printing medium. Control of the relative speed between the ink-jet printhead and the printing medium, control of the timing of ink discharge that accompanies this movement, and the stability with which the printhead is supplied with power are factors that influence the image quality of the printed result.
An ink-jet printing apparatus may be classified broadly into two types, namely so-called serial type and full-line type, depending upon the mode of the ink-jet printhead. The serial-type apparatus of these performs printing by discharging ink while the ink-jet printhead is moved (scanned) relative to the printing medium. The serial-type apparatus is in wide use because of the simplicity of its structure.
Types of printhead that discharge ink include one that discharges the ink by the operation of piezoelectric elements and one that discharges the ink by causing the ink to undergo film boiling momentarily. A printhead of the type that discharges ink by the film boiling of ink is so adapted that by passing a current into a heater provided in the vicinity of ink passageway near an ink orifice, ink in the vicinity of the heater is cause to undergo film boiling to thereby provide the discharge energy.
Uniform ink droplets are obtained by arranging it so that the energy for discharging the ink is supplied stably at all times and so that the ink discharge will take place under the same conditions. This is important in terms of maintaining an excellent print image quality. However, since the frequency (duty ratio) with which ink is discharged differs depending upon the print data in the printing operation, the number of heaters energized simultaneously differs from time to time. As a consequence, a change occurs in the driving conditions, in accordance with the image data to be printed, owing to such effects as a fluctuation in voltage caused by a difference in the value of current that is output from a power supply, and a difference in a voltage drop ascribable to the resistance component of the transmission system.
In order to stabilize such driving conditions, it has been contrived heretofore to raise the precision of the output voltage from the power source or to adopt an arrangement that minimizes loss in the transmission system.
A DC/DC converter that supplies power to the header of a printhead will be described next.
In order to cause film boiling in ink momentarily by passing a current into the heater, as mentioned above, it is required that the heater be supplied with a high voltage. To achieve this, a DC/DC converter is used for the purpose of generating a voltage higher than the power-supply voltage of the other circuitry.
FIG. 7 is a circuit diagram illustrating an example of the structure of the conventional DC/DC converter and its voltage control circuit. The input voltage Vin of the DC/DC converter supplied from a power supply unit (not shown) is applied to a switching element 201. A DC output obtained by a conversion in the switching element 201 and diode 209 is delivered via an inductance 202 and is supplied as an output voltage VH-b to a printhead serving as a load.
A capacitor 203 is connected to the input side of the switching element 201. Further, a capacitor 204 is connected via the inductance 202 and a smoothing circuit 205 is constructed by the inductance 202 and capacitor 204. The output voltage signal VH-b, which is detected from the output terminal of the smoothing circuit 205, is input to a current control circuit 206 upon being voltage-divided by resistors R1 and R2, and feedback of the signal is controlled by an error amplifier 207 constituting the voltage control circuit 206.
A potential obtained by voltage-dividing a reference voltage Vref by resistors R3, R4 and a potential obtained by voltage-dividing the feed-back output voltage VH-b by the resistors R1, R2 are input to the error amplifier 207. The output signal of the error amplifier 207, which is the output signal of the voltage control circuit 206, controls the switching element 201 to a constant voltage via a PWM comparator 208. A resistor R5 and a capacitor C1 connected between an inverting terminal and the output terminal of the error amplifier 207 represent one example of a phase compensating circuit.
Thus, the output of the DC/DC converter is subjected to feedback control so as to supply a stable output voltage irrespective of the value of output current, which varies owing to a change in the number of nozzles driven simultaneously by the printhead constituting the load.
It has become possible in recent years to handle color images with facility owing to the higher speeds of computers and the like. In addition, as a result of the greater number of pixels usable in image input devices such as digital cameras, there is much greater demand for higher image quality and higher speed in ink-jet printers used as the output devices. By way of example, an increase in the speed of the printing operation of an ink-jet printer can be achieved by raising the discharge (driving) frequency and by increasing the number of nozzles that are driven simultaneously. It is possible to achieve both an improvement in image quality and higher speed by discharging the ink from each nozzle as extremely small liquid droplets and increasing the amount of ink discharged per unit time.
Consider a case where printing speed is raised by increasing the number of nozzles driven simultaneously. Among the nozzles arranged so as to be capable of being driven simultaneously, of course the number of nozzles actually driven to discharge ink changes depending upon the image recorded at the time. When an all-black image is printed, for example, it is necessary to discharge ink simultaneously from all of the nozzles that are capable of discharging ink. On the other hand, if the image is one having a low duty ratio, such as a ruled line, only some of the nozzles need discharge ink simultaneously.
As mentioned above, the printing operation, i.e., ink discharge, of an ink-jet printhead in a serial printer is achieved by thermal energy produced by passing a current into heaters.
Since electric current is necessary for the ink discharge, the value of current required increases in proportion to the increase in number of nozzles that discharge ink simultaneously. Further, with a serial printer, the number of nozzles that are driven simultaneously is not always fixed and the number varies successively in accordance with the print data sent to the printhead.
In other words, in an ink-jet printer that forms images, designs, patterns and characters, etc., on a printing medium in accordance with image data transmitted from an external device, the amount of ink discharged from the printhead per unit time is decided by the image data sent from the external device, and the power consumed by the printhead per unit time is decided by the amount of ink discharged per unit time.
That is, the greater the amount of image data, the greater the number of nozzles driven simultaneously and the greater the power consumed by the printhead. Conversely, if the amount of image data per unit time is small, the number of nozzles driven simultaneously is small and the power consumed by the printhead is small. Thus, the amount of current that the DC/DC converter supplies to the printhead is decided in proportion to the number of nozzles driven simultaneously.
With regard to an improvement in the characteristic of voltage control in a DC/DC converter, the specification of Japanese Patent Application Laid-Open No. 6-233530 discloses a method of inserting a resistor in the output line, detecting load current and improving response when there is a sudden change in load. However, because a voltage drop is produced by the resistor inserted into the output line, it is difficult to apply this method to a DC/DC converter that supplies power to the ink-jet printhead, which requires a voltage of high precision. Furthermore, since a loss in power due to the inserted resistor also occurs, the power conversion efficiency deteriorates. Accordingly, this method is not desirable in a small-size DC/DC converter.
The power supply that supplies the printhead with power will now be described. In order to diminish a fluctuation in output power with respect to a change in current caused by a change in the number of nozzles driven simultaneously in the printhead constituting the load, the value of a steady gain K in the voltage feedback control loop is enlarged.
When the value of the steady gain K is increased, however, not only is stability lost in the unloaded state but there is also the danger that problems will arise owing to non-linearity of the PWM control loop.
The value of the steady gain K cannot be made very large for the foregoing reasons. The conventional voltage control loop, therefore, cannot deal adequately with a momentary current fluctuation ascribable to a fluctuation in load and the transient fluctuation characteristic of output voltage deteriorates. The drop in output voltage with respect to a momentary current fluctuation is suppressed by a capacitative component typified by an electrolytic capacitor, which converts the momentary current to an average current, inserted into the output end of the circuit.
The power supply of an ink-jet printer in which the driving conditions of the printhead that discharges ink droplets change greatly depending upon the image data transmitted from an external device must be designed so as to supply an output voltage stably with respect to all momentary load fluctuations taking into account also the driving conditions of the printhead. However, since the value of the gain K cannot be made sufficiently large, a constant-voltage control circuit cannot follow up a momentary current fluctuation that occurs suddenly from the unloaded state up to a rated maximum current value at which drive, which is the fully loaded state, is achieved. As a result, the power supply is designed so as to suppress the output voltage fluctuation by enlarging the capacitance of the capacitative component typified by an aluminum electrolytic capacitor or the like inserted into the output end of the power supply means.
As mentioned above, in a case where load fluctuates sharply from the unloaded state, in which there is no ink discharge, to the maximum load current for discharging ink from all nozzles of the printhead, the above-described voltage control circuit that relies upon error amplification using the potential difference between a reference voltage and the output voltage is such that the voltage control circuit can no longer follow up owing to a delay in the constant-voltage feedback (the amount of error voltage in the error amplifier) of the DC/DC converter, and hence voltage drops below the set voltage. It is necessary to make the capacity of the output capacitor fairly large in order to prevent this drop in output voltage. However, this is an impediment as far as a reduction in the size and thickness of the DC/DC converter is concerned.
Furthermore, in order to achieve higher speed and better image quality, there is a tendency for the printhead of the ink-jet printer to be made longer and to be provided with a greater number of nozzles so that the number of nozzles driven simultaneously increases. This means that the maximum output current of the load takes on a large value. A method other than the method of enlarging the capacity of a capacitor is needed to suppress the drop in output voltage at the time of a sudden change in load.